1. Field of the Invention
This invention relates to gear mechanisms, and in particular to fluid motors or pumps of the progressive cavity, or Moineau, type.
2. Description of the Prior Art
U.S. Pat. No. 1,892,217 (Moineau) describes a gear mechanism of the Moineau type. This type of mechanism may be used either as a pump or as a fluid motor. The mechanism has two helical gear members disposed within one another. The outer gear member has one helical thread more than the inner gear mechanism. Forcing fluid through the outer gear mechanism will cause the inner mechanism to rotate.
The outer gear mechanism is generally a resilient sleeve, sealingly mounted within a metal body. The interface between the body and the sleeve may be cylindrical or helical. When the interface is helical, the sleeve is usually of a constant thickness, as shown in U.S. Pat. No. 3,084,631 (Bourke).
In U.S. Pat. No. 4,104,089 (Chanton), bosses are added to the inner and outer surfaces of the sleeve. The bosses are located in those areas which correspond to to the highest sliding speeds.
Downhole motors are often used to drill oil wells. In downhole motors of the Moineau type, the outer gear member is a stator and the inner member is a rotor. There must be an interference fit between the rotor surface and the stator surface to provide a pressure seal between the motor stages.
The rubbing of the rotor in the stator, especially in a drilling mud environment, causes the stator surface to wear. The interference and the amount of pressure sealed between the motor stages is thus reduced. A thick resilient sleeve allows much interference between the rotor and the stator, and allows considerable wear of the stator before the pressure seal is reduced to an unacceptable level.
A pressure drop is required across the motor and individually across the motor stages in order to overcome external resisting torque. This places stresses on the resilient sleeve that cause fatigue or hysteresis failures.
The rubbing of the rotor on the stator and the stresses on the stator also cause heat to build up. This heat can also cause the resilient sleeve to break down.